Bainitic steel rail containing trace amounts of carbides and producing method of the same

ABSTRACT

The present invention discloses a bainitic steel rail containing trace amounts of carbides, wherein, the bainitic steel rail mainly consists of bainitic structures, the carbides have a length of 0.05-0.5 μm, the major axis of carbides is oriented to a direction at a 50-70° included angle from the direction of the major axis of bainitic ferrite plates, and the carbides account for 1%-5% by volume. The present invention further discloses a method for producing a bainitic steel rail containing trace amounts of carbides, comprising: cooling a steel rail with residual heat after finish rolling by air cooling, till the temperature at the center of the rail head tread reaches 420-450° C.; cooling the rail head part of the steel rail by accelerated cooling at a 2.0-5.0° C./s cooling rate, till the temperature at the center of the rail head tread reaches 220-240° C.; loading the steel rail into a tempering furnace and tempering at 300-350° C. for 4-6 h; then, cooling the steel rail by air cooling to the room temperature. The steel rail provided in the present invention has outstanding wear resistance and contact fatigue resistance properties, and can meet higher requirement for the service of railroad steel rails, and the product is especially applicable to heavy-duty railroads.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Application No. 201410192490.X, entitled “Bainitic Steel Rail Containing Trace amounts of Carbides and Producing Method of the Same”, filed on May 8, 2014, which is specifically and entirely incorporated by reference.

FIELD OF INVENTION

The present invention relates to a bainitic steel rail containing trace amounts of carbides and a method for producing the bainitic steel rail containing trace amounts of carbides.

BACKGROUND

At present, most of the steel rails widely used for railroads are made of eutectoid steel, the microscopic structure of which mainly consists of pearlites and contains trace amounts of ferrites, characterized in good toughness matching and moderate performance, etc. However, as the traffic density and axle load on the rails of railroad are increased persistently, the existing steel rail products can't meet the requirement for service on the lines any more, in particular in railroad sections where the line conditions are harsh. Wherein, the extremely quick wearing of the wheel-rail contact part has gradually become a major factor that affects the service life of the steel rails on heavy-duty railroads, especially the steel rails at parts with a sharp radius curve. To solve that problem, the developers in the art have been making efforts to develop new steel rail products that have a better wear resistance property as well as outstanding over-all properties such as contact fatigue resistance, to meet the demand in railroad engineering.

Through years of research, it is found that there are mainly two methods that can meet the above-mentioned requirement: one method is to further increase the carbon content in the rail steel and add alloying elements in an appropriate amount as supplement, to give full play to the wear resistance improving effect of carbon in the steel rails, and obtain better toughness matching and over-all properties of the steel rails through a post-rolling cooling process; the other method is to utilize bainitic steel rails with high alloy content and obtain bainitic steel rails with a high wear resistance property by controlling a post-rolling cooling process, so as to improve the wear resistance property while giving full play to the outstanding contact fatigue resistance. It is proved in practice that further increasing the carbon content in the existing steel rail products will bring detrimental effects to the safety of application of the steel rails incurred by inadequate toughness and plasticity and secondary cementite precipitation. In recent years, the practice of applying bainitic steel for railroad steel rails brought a new idea to the development of new steel rail products. However, as for existing bainitic steel rails, it is difficult to solve the problem of realizing high wear resistance while maintaining the outstanding contact fatigue resistance property of the steel rails. For example, in the case of the bainitic steel rail disclosed in the patent document CN1074058C, the rail head part has 230-320 of Vickers hardness, and can't effectively resist the wheel-rail wearing owing to the low hardness; consequently, the steel rail has to be replaced early before it reaches its design life, because it is worn severely. The steel rails disclosed in the patent documents CN1101856C, CN1219904C, and CN1012906B, etc. are similar to the steel rail described above. In patent document CN1086743C, a bainitic steel rail that has high surface fatigue damage resistance and high wearing resistance performance is disclosed. The microscopic structure of the bainitic steel rail is characterized in: based on the total area of a given cross section of the bainitic structure, the total area of carbides with a major axis within 100 nm-1,000 nm range accounts for 10-50%. That technique has the following obvious drawbacks: as a hard phase in the steel, the carbides account for a percentage that is too high, and consequently the cracks formed in the steel rail develop preferentially along the carbides under stress, resulting in fatigue damages such as cracking and peeling, or even fractures of the steel rail which will endanger operation safety. Though measures have been taken to decrease the sizes of the carbides in the invention to avoid the above problems, those problems still can't be solved effectively from the root because the percentage of the carbides is too high. In patent documents CN100471974C and CN1166804C, a bainitic steel rail under an air cooling condition and the producing method of the same are disclosed. The producing procedures for the bainitic steel rail are quite different to those in the present invention, because the method employs air cooling after rolling.

In summary, as for the bainitic steel rails and the producing methods of the same that have been disclosed up to now, though the contact fatigue resistance of the bainitic steel rails is given full play, the problem of wear resistance of the bainitic steel rails has not been solved from the root. There is an urgent need for a bainitic steel rail that has outstanding wear resistance and fatigue damage resistance properties, to meet the requirement for service on heavy-duty railroads, in particular in railroad sections where the conditions are harsh.

SUMMARY

To overcome a drawback of existing steel rails, i.e., outstanding wear resistance and fatigue damage resistance properties can't be realized at the same time in these steel rails, the present invention provides a bainitic steel rail that has outstanding wear resistance and fatigue damage resistance properties and a producing method of the same.

Similar to the case of the carbides in pearlitic steel rails, the size and percentage of the carbides in bainitic steel rails have apparent influences on the wear resistance and service life of the steel rails. In the service process, the steel rail is subjected to the reciprocating action of complex stress exerted by the wheels, and the rail head wheel-rail contact part of steel rail will be worn continuously by the friction force created between the steel rail and the wheels. Analyzed microscopically, the bainitic ferrite in the steel rail is a soft phase in the steel, and it still can not have enough strength to resist the wearing by the wheels though it has been strengthened in the accelerated cooling process after rolling. Whereas, the carbides, which is a hard phase in the steel, will gradually precipitate from the bainitic ferrite and concentrate as the surface layer of rail head is worn in the service process, and thereby will resist the stress exerted by the wheels together and can improve the wear resistance of the steel. In the research, the inventor of the present invention has found: the carbides that precipitate from the bainitic ferrite matrix are in rod-shaped or strip-shaped, in length of not greater than 0.5 μm, and are oriented to a direction at a 50-70° included angle from the direction of the major axis of ferrite plates, and these carbides can effectively improve the wear resistance property of the steel rail, with little adverse effect to the rolling contact fatigue resistance property of the steel rail.

To attain the object described above, in one aspect, the present invention provides a bainitic steel rail containing trace amounts of carbides, wherein, the bainitic steel rail mainly consists of bainitic structures, the carbides have a length of 0.05-0.5 μm, the major axis of carbides is oriented to a direction at a 50-70° included angle from the direction of the major axis of bainitic ferrite plates, and the carbides account for 1%-5% by volume.

In another aspect, the present invention further provides a method for producing a bainitic steel rail containing trace amounts of carbides, comprising: cooling a steel rail with residual heat after finish rolling by air cooling, till the temperature at the center of the rail head tread reaches 420-450° C.; cooling the rail head part of the steel rail by accelerated cooling at a 2.0-5.0° C./s cooling rate, till the temperature at the center of the rail head tread reaches 220-240° C.; loading the steel rail into a tempering furnace and tempering at 300-350° C. for 4-6 h; then, cooling the steel rail by air cooling to the room temperature.

Other characteristics and advantages of the present invention will be further detailed in the embodiments hereunder.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is provided to facilitate further understanding on the present invention, and constitutes a part of the specification. It is used in combination with the following embodiments to explain the present invention, but shall not be comprehended as constituting any limitation to the present invention.

The FIGURE shows the microscopic structure of the bainitic steel rail containing trace amounts of carbides provided in the present invention under a transmission electron microscope (TEM) after the thickness is reduced by twin-jet electropolishing.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereunder some embodiments of the present invention will be detailed, with reference to the accompanying drawing. It should be appreciated that the embodiments described here are only provided to describe and explain the present invention, but shall not be deemed as constituting any limitation to the present invention.

In one aspect, the present invention provides a bainitic steel rail containing trace amounts of carbides, wherein, the bainitic steel rail mainly consists of bainitic structures, the carbides have a length of 0.05-0.5 μm, the major axis of carbides is oriented to a direction at a 50-70° included angle from the direction of the major axis of bainitic ferrite plates, and the carbides account for 1%-5% by volume.

In the research, the inventor of the present invention has found: if the percentage of carbides is too high, e.g., higher than 5%, though the wear resistance property of the steel rail can be improved effectively, cracks may occur easily in the steel rail under the stress exerted by the wheels and the steel rail, and the cracks will developed preferentially along the carbides in the steel; consequently, the steel rail will fracture in a short time, and the service safety can't be ensured. The major axis direction of the carbides is at 50-70° included angle from the major axis of the bainitic ferrite plates, which is advantageous for ensuring the fatigue damage resistance property of the steel rail will not be degraded even after plastic deformation occurs at the wheel-rail contact part.

According to the present invention, the bainitic steel rail contains: 0.15 wt %-0.30 wt % C, 1.00 wt %-1.80 wt % Si, 1.50 wt %-2.50 wt % Mn, 0.50 wt %-1.00 wt % Cr, 0.20 wt %-0.50 wt % Mo, and Fe that accounts for the remaining content, and the total weight percentage of Mn and Cr meets 2.2%≦Mn+Cr≦3.0%.

Hereunder the reasons for confining the major chemical elements in the steel rail disclosed in the present invention within the above ranges will be explained:

Carbon (C) is the most important element for bainitic steel to obtain outstanding toughness matching and overall mechanical properties. When the carbon content is lower than 0.15 wt %, it is unable to give full play to the strengthening effect, and the rigidity of the steel rail will be too low, and consequently the carbide percentage in the steel and the wear resistance property of the steel can't be ensured; when the carbon content is higher than 0.30 wt %, through the process described in the present invention, the strength of the steel is too high, while the toughness and plasticity are too low; consequently, the contact fatigue resistance of the steel is severely affected because the percentage of carbides is too high, and the application safety of the steel rail is compromised. Therefore, the carbon content is confined to 0.15-0.30 wt %.

As a major additional element in steel, silicon (Si) usually exists in ferrite in the form of solid solution, and can improve the strength of the structure. For bainitic steel, when the silicon content is lower than 1.00 wt %, on the one hand, the strengthening effect is not significant because the concentration of solid solution is low; on the other hand, fine carbides can't be obtained, and consequently the structural control objective of the present invention can't be attained; when the silicon content is higher than 1.80 wt %, the carbide precipitation will be fully inhibited; instead, residual austenite will exist, and surface defects may easily occur, consequently, the smoothness of train operation can't be ensured. Therefore, the silicon content is confined to 1.00-1.80 wt %.

Manganese (Mn) can significantly decrease the initial transition temperature of bainitic structure, improve the hardness of carbides, and is an important additional element in bainitic steel. In the research, the inventor has found: when the manganese content is lower than 1.50 wt %, it will be difficult to attain the effect of improving carbide hardness; when the manganese content is higher than 2.50 wt %, the carbide hardness will be too high, and the fatigue resistance property of the steel rail will be severely degraded. Therefore, the manganese content is confined to 1.50-2.50 wt %.

As a forming element for moderate-size carbides, chromium (Cr) can bond with carbon in steel to form a variety of carbides; in addition, chromium is helpful for uniform distribution of carbons in steel, and can decrease carbide size, and thereby improve the wear resistance property of the steel rail. When the chromium content is lower than 0.50 wt %, the hardness and percentage of the carbides formed in steel will be low, and the carbides will concentrate in the form of flakes, which is adverse to the service performance of the steel rail; when the chromium content is higher than 1.00 wt %, the percentage of martensite in steel will be increased severely, and consequently the service safety of the steel rail can't be ensured. Therefore, the chromium content is confined to 0.50-1.00 wt %.

Molybdenum (Mo) has a remarkable effect for decreasing the initial transition temperature of bainitic structure, and is advantageous for stabilizing and strengthening bainitic structure. When the molybdenum content is lower than 0.20 wt %, it will be difficult to attain the above-mentioned effects; when the molybdenum content is higher than 0.50 wt %, the transition efficiency of bainitic structure will be severely decreased, and consequently an ideal bainitic structure can't be obtained in the accelerated cooling process. Therefore, the molybdenum content is confined to 0.20-0.50 wt %.

To further improve the service performance of the steel rail in the present invention, the manganese content and chromium content should meet 2.2 wt %≦Mn+Cr≦3.0 wt %. Mn and Cr have similar effects in bainitic steel; when Mn+Cr<2.20 wt %, the strength, size, and percentage of carbides in steel can't meet the requirement described in the present invention; in addition, the hardness of carbides is low, and a moderate wear resistance property can't be obtained; when Mn+Cr>3.00 wt %, on the one hand, the hardness of carbides will be too high; on the other hand, severe segregation will occur locally in the steel rail, and consequently the uniformity of bainitic structure and the performance of bainitic structure in the steel rail can't be ensured. Therefore, a condition of “2.2 wt %≦Mn+Cr≦3.0 wt %” must be met. Here, “Mn+Cr” refers to the sum of Mn content and Cr content.

In another aspect, the present invention further provides a method for producing a bainitic steel rail containing trace amounts of carbides, comprising: cooling a steel rail with residual heat after finish rolling by air cooling, till the temperature at the center of the rail head tread reaches 420-450° C.; cooling the rail head part of the steel rail by accelerated cooling at a 2.0-5.0° C./s cooling rate, till the temperature at the center of the rail head tread reaches 220-240° C.; loading the steel rail into a tempering furnace and tempering at 300-350° C. for 4-6 h; then, cooling the steel rail by air cooling to the room temperature.

According to the method provided in the present invention, a steel rail with residual heat after finish rolling is utilized, the rail head part of the steel rail is cooled by accelerated cooling at a 2.0-5.0° C./s cooling rate; when the temperature at the center of the rail head tread drops to 220° C.-240° C., the steel rail is loaded into a tempering furnace and is tempered at 300-350° C. for 4-6 h; then, the steel rail is cooled to the room temperature by air cooling; in that way, rod-shaped carbides can precipitate from the bainitic ferrite matrix, wherein the rod-shaped carbides are in 0.05-0.5 μm length and oriented to a direction at a 50-70° included angle from the major axis of the bainitic ferrite plates, and account for 1%-5% by volume.

According to the method provided in the present invention, the steel rail with residual heat after finish rolling can be produced with a common method in the art; for example, the method can comprise: processing a steel material with appropriate chemical composition by smelting in a converter or electric furnace, LF refining, RH or VD vacuum treatment, and casting, to produce a continuous casting steel billet with appropriate cross sectional dimensions; then, the steel billet is loaded into a walking beam furnace and heated up to 1200-1300° C., and is held at that temperature for 2 h or longer time; next, the steel billet is rolled to a steel rail with required cross sectional dimensions; here, the finish rolling temperature of the steel rail is 850-950° C.

The rail steel with residual heat after finish rolling is erected on a roller way and held in the air for air cooling; when the temperature of the surface layer of steel rail head drops to 420-450° C., an accelerated cooling medium is applied to the top surface and both sides of the rail head. Here, the accelerated cooling medium can be a commonly used cooling medium in the art. For example, the accelerated cooling medium can be selected from at least one of compressed air, water-air mixture, and oil-gas mixture.

Hereunder the reason for setting the initial accelerated cooling temperature to 420-450° C. will be explained. As indicated in the research by the inventor of the present invention, under the post-rolling air cooling condition, the phase transition temperature of bainitic steel rails is usually within the range of 350-400° C. If the accelerated cooling is initiated from the temperature range of austenitic phase region, longer cooling time will be required and more cooling medium energy will be consumed, since the initial accelerated cooling temperature is far away from the phase transition temperature; more importantly, in the accelerated cooling process, the heat from the core part of rail head and the rail web part will diffuse towards the surface layer of rail head by heat transfer, while the surface layer of rail head is subjected to accelerated cooling by the external cooling medium; consequently, the rail head part can't accomplish phase transition at a higher super-cooling degree, ultimately the rigidity on the cross section of rail head will decrease gradually from the surface layer to the core part, and the steel rail can't be hardened entirely. By setting the initial cooling temperature as 420-450° C., the following benefit can be obtained: initiating the accelerated cooling in the temperature range from the austenitic phase region to 450° C. has little contribution to the improvement of the overall performance of the steel rail. When the steel rail is cooled to 420-450° C., both the temperature of the rail web and the temperature of the rail base are lower than 480° C. If the accelerated cooling is initiated at that temperature, the temperature of the surface layer of rail head will be decreased significantly, while the heat from the core part of rail head is difficult to effectively replenish the heat loss in the surface layer; in addition, since the initial cooling temperature is close to the phase transition point, the entire cross section of rail head, in particular to core part of rail head, can accomplish phase transition at a higher super-cooling degree. The reason why the cooling rate is set to 2.0-5.0° C./s in that process is as follows: if the cooling rate is lower than 2.0° C./s, the temperature of the surface layer of rail head can't be cooled down quickly, and the cooling effect can't be transferred effectively to the core part; in addition, the heat from the core part will anti-replenish to the surface layer, which is adverse to the improvement of the overall performance of the steel rail; more importantly, the carbides in the steel rail can't precipitate sufficiently, and consequently the object of the present invention can't be attained; if the cooling rate is higher than 5.0° C./s, more martensite will be produced because the surface layer is cooled down too quickly, consequently, the rigidity of the steel rail will be too high; though the martensite can be converted to tempered martensite partially through the follow-up tempering process, residual martensite will remain and form martensitic structures at the room temperature finally, which will be adverse to the safe application of the steel rail.

The accelerated cooling is stopped when the surface layer of steel rail is cooled down to 220-240° C. The reason why the final temperature of accelerated cooling is set to 220-240° C. is as follows: if the final cooling temperature is higher than 240° C., though fine bainitic structures have been obtained in the surface layer of rail head, coarse bainitic structures will be formed in the core part of rail head owing to the high temperature, and the coarse bainitic structures will influence the performance of the steel rail at the room temperature and will be adverse to the uniformity of performance of the entire cross section; if the cooling temperature is lower than 220° C., a large quantity of martensite will be formed, and can't be eliminated even through the follow-up tempering treatment; consequently, the toughness and plasticity of the steel rail will be severely compromised, or even the steel rail can't be used.

In addition, after the accelerated cooling is completed, the steel rail is loaded into a heating furnace and tempered at 300-350° C. for 4-6 h, and then is cooled to the room temperature by air cooling. The reason for the above setting is: if the tempering temperature is lower than 300° C., the toughness and plasticity of the steel, especially the impact toughness at a low temperature, will be severely degraded, consequently, the high toughness property of bainitic steel rail at a low temperature can't be utilized; in addition, since the carbides can't precipitate sufficiently from the steel, the wear resistance property of the steel rail can't be improved; if the tempering temperature is higher than 350° C., though the toughness and plasticity still increase, the strength and hardness will decrease, consequently, it will be difficult to obtain a steel rail with outstanding overall properties. The reason why the tempering time is set to 4-6 h is: when the tempering time is shorter than 4 h, the carbides in the steel, in particular the carbides in the deep zone of the rail head, can't precipitate sufficiently; when the tempering time is longer than 6 h, the excessively long processing time will bring little benefit, because the carbide precipitation in the steel has already completed and the objective of the tempering process has already attained. After the tempering treatment, the steel rail is taken out and cooled to the room temperature by air cooling, so as to obtain a finished steel rail product.

According to the method provided in the present invention, the steel rail contains: 0.15 wt %-0.30 wt % C, 1.00 wt %-1.80 wt % Si, 1.50 wt %-2.50 wt % Mn, 0.50 wt %-1.00 wt % Cr, 0.20 wt %-0.50 wt % Mo, and Fe that accounts for the remaining content, and the total weight percentage of Mn and Cr meets 2.2%≦Mn+Cr≦3.0%.

EXAMPLES

Hereunder the present invention will be detailed in some examples, but the scope of the present invention is not limited to those examples.

In examples 1-6 and Comparative Examples 1-6, the following steel rails 1-6 are used respectively. The chemical compositions of the steel rails are shown in Table 1.

TABLE 1 Chemical Composition/wt % No. C Si Mn P S Cr Mo Mn + Cr 1 0.23 1.58 1.97 0.010 0.006 0.80 0.20 2.77 2 0.20 1.20 2.50 0.011 0.005 0.50 0.29 3.00 3 0.15 1.80 1.50 0.011 0.007 1.00 0.42 2.50 4 0.21 1.45 1.60 0.014 0.009 0.60 0.50 2.20 5 0.24 1.00 2.05 0.012 0.004 0.63 0.36 2.68 6 0.30 1.30 1.87 0.013 0.006 0.78 0.25 2.65

Example 1

Process the steel No. 1 in Table 1 by converting in a converter, LF refining, RH vacuum treatment, and casting, to produce a continuous casting steel billet; load the steel billet into a walking beam furnace and heat up to 1300° C. and hold for at least 2 h; roll the steel billet into a 60 Kg/m steel rail; after finish rolling, erect the steel rail on a roller way and hold the steel rail there by means of a steel turnover rack for air cooling, till the temperature at the center of the rail head tread reaches 445° C.; next, apply a cooling medium to the top surface and two sides of the rail head to start accelerated cooling, wherein, the cooling medium is water-air mixture, and the steel rail is cooled at 4.5° C./s accelerated cooling rate, till the temperature of the surface layer of rail head drops to 240° C.; then, stop the accelerated cooling, load the steel rail into a tempering furnace and temper at 300° C. for 4.1 h. After the tempering treatment, cool the steel rail in the air to the room temperature; thus, a steel rail A1 is obtained finally.

Examples 2-6 and Comparative Examples 1-6

Prepare steel rails in examples 2-6 with the method described in Example 1, but replace the control parameters in the operating process in Example 1 with those shown in Table 2. The steel rails prepared with the method in Examples 2-6 are denoted as A2-A6. In the Comparative Examples, the processing method is a conventional thermal processing method and the specific control parameters in the operating process are shown in Table 2. The steel rails prepared with the method in Comparative Examples 1-6 are denoted as D1-D6.

TABLE 2 Initial Final Temperature Cooling Rate Temperature Tempering of Accelerated of Accelerated of Accelerated Temperature/ Tempering Item No. Cooling/° C. Cooling/° C./s Cooling/° C. ° C. Time/h Examples 1 445 4.5 240 300 4.1 2 432 3.0 235 338 4.8 3 429 3.4 232 350 6.0 4 420 2.0 227 344 5.1 5 448 5.0 220 330 4.0 6 450 4.1 224 340 5.6 Comparative 1 760 1.8 350 — — Examples 2 780 2.4 381 — — 3 820 2.2 364 — — 4 880 3.1 425 — — 5 690 2.9 346 — — 6 870 1.9 315 — —

Test Examples

The performances of the steel rails A1-A6 prepared in Examples 1-6 and D1-D6 prepared in Comparative Examples 1-6 are tested with the following method, specifically:

The tensile property of the steel rail is measured according to GB/T228-2010 “Tensile Testing Method of Metallic Materials at Room Temperature”, and the measured Rp0.2 (stress at 0.2% of non-proportional elongation), Rm (tensile strength), A % (elongation), Z % (reduction of cross section) are shown in Table 3;

Wearing tests are carried out on a MM-200 wear testing machine to test the average weight loss resulted from wearing. The samples are taken from the rail head part of steel rails A1-A6 and D1-D6. In all of the wearing tests, the lower grinding samples are made of the same material. The measured values of average weight loss resulted from wearing are shown in Table 3. The testing parameters are as follows:

-   -   Sample size: round sample in 10 mm thickness and 36 mm diameter     -   Test load: 150 Kg     -   Slippage: 10%     -   Material of lower grinding sample: wheel steel with hardness of         260-310 HB     -   Environment: in air     -   Rotation speed: 200 rpm     -   Total wearing cycles: 100,000 cycles

The carbide length, the included angle between the carbides and the bainitic ferrite, and the percentage of the carbides are measured with the following method:

Samples are taken from the steel rails prepared in the Examples and Comparative Examples and film samples in thickness of ≦50 μm are obtained from the samples. Then, the samples are processed by two-jet electropolishing for thickness reduction; next, the carbide morphology is indexed and observed under a transmission electron microscope (TEM), and the included angle between the carbides and the bainitic ferrite is measured; carbides in 0.05-0.5 μm length and oriented to a direction at a 50-70° included angle are selected, and the carbide area and percentage are measured by approximate estimation. Since the morphology of the carbides varies in different viewing fields, in order to ensure the accuracy of measurement, at least 20 viewing fields on a steel rail with same material, same process and same sampling position are observed, and the average value is taken, and the percentage of carbides that meet the requirement is determined.

TABLE 3 Impact Property Average Weight Loss Tensile Property Aku/J Percentage Length of Resulted R_(p0.2)/ R_(m)/ Room of Carbides/ Carbides/ from Item No. MPa MPa A/% Z/% Temperature 0° C. vol. % μm Wearing/g Examples A1 1230 1480 16.5 52 95 78 2.8 0.42 0.5466 A2 1280 1510 15.5 48 85 60 4.4 0.08 0.5143 A3 1150 1430 18.0 58 107 81 1.8 0.26 0.5896 A4 1290 1590 16.5 50 98 64 4.2 0.32 0.4831 A5 1260 1490 17.0 52 92 72 3.5 0.40 0.4269 A6 1360 1610 15.0 44 78 56 4.9 0.29 0.3987 Comparative D1 1025 1340 16.0 52 75 48 N/A — 1.0236 Examples D2 1040 1350 15.0 44 54 38 N/A — 0.9584 D3 1080 1290 17.5 49 52 40 N/A — 1.1459 D4 1105 1420 16.5 40 58 40 N/A — 0.8562 D5 1060 1310 15.0 46 68 46 N/A — 0.7569 D6 1180 1480 14.0 40 66 41 N/A — 0.7258

The results in Table 3 indicate: under the condition of the same chemical composition and the same smelting and rolling process, the treatment of the steel rail of post-rolling will have significant influence on the final properties of the steel rail, represented by: in the steel rail produced with the method disclosed in the present invention, rod-shaped or strip-shaped carbides, which are in 0.05-0.5 μm length, oriented to a direction at a 50-70° included angle from the direction of the major axis of the ferritic plates, and account for 1-5 vol. %, precipitate from the bainitic ferrite matrix; as a result, the steel rail obtains excellent toughness and significantly improved wear resistance property under the same conditions. Hence, the present invention is helpful for prolonging the service life of the steel rails, especially the steel rails of curve road section with harsh line conditions on heavy-duty railroads.

Preferred embodiments of the present invention are described above in detail, however, the present invention is not limited to the specific details of the above embodiments, technical solutions of the present invention may have various simple modifications within the technical spirit of the present invention, and these simple modifications belong to the scope of the present invention.

In addition, it should be noted that each specific technical characteristic described in the above specific embodiments can be combined in any suitable manner, without contradictory situation. In order to avoid unnecessary repetition, various possible combinations are not further explained in the present invention.

Moreover, various embodiments of the present invention may also be combined in any suitable manner, as long as it will not depart from the idea of the present invention, and the combinations should be regarded as the disclosure of the present invention. 

What is claimed is:
 1. A bainitic steel rail containing trace amounts of carbides, wherein, the bainitic steel rail mainly consists of bainitic structures, the carbides have a length of 0.05-0.5 μm, the major axis of carbides is oriented to a direction at a 50-70° included angle from the direction of the major axis of bainitic ferrite plates, and the carbides account for 1%-5% by volume.
 2. The bainitic steel rail according to claim 1, wherein, the bainitic steel rail contains: 0.15 wt %-0.30 wt % C, 1.00 wt %-1.80 wt % Si, 1.50 wt %-2.50 wt % Mn, 0.50 wt %-1.00 wt % Cr, 0.20 wt %-0.50 wt % Mo, and Fe that accounts for the remaining content, and the total weight percentage of Mn and Cr meets 2.2%≦Mn+Cr≦3.0%.
 3. A method for producing the bainitic steel rail containing trace amounts of carbides in claim 1, comprising: cooling a steel rail with residual heat after finish rolling by air cooling, till the temperature at the center of the rail head tread reaches 420-450° C.; cooling the rail head part of the steel rail by accelerated cooling at a 2.0-5.0° C./s cooling rate, till the temperature at the center of the rail head tread reaches 220-240° C.; loading the steel rail into a tempering furnace and tempering at 300-350° C. for 4-6 h; then, cooling the steel rail by air cooling to the room temperature.
 4. The producing method according to claim 3, wherein, the medium of the accelerated cooling is selected from at least one of compressed air, water-air mixture, and oil-gas mixture.
 5. The producing method according to claim 3, wherein, the steel rail contains: 0.15 wt %-0.30 wt % C, 1.00 wt %-1.80 wt % Si, 1.50 wt %-2.50 wt % Mn, 0.50 wt %-1.00 wt % Cr, 0.20 wt %-0.50 wt % Mo, and Fe that accounts for the remaining content, and the total weight percentage of Mn and Cr meets 2.2%≦Mn+Cr≦3.0%. 